Human defence systems against disease

What you'll learn

This topic covers how the human body prevents pathogens from entering and how it fights infections once pathogens do enter. You'll study the body's non-specific defence mechanisms, the role of white blood cells in immunity, and how vaccination provides protection against specific diseases. This content is essential for AQA GCSE Biology Paper 2.

Key terms and definitions

Pathogen — a microorganism that causes disease, including bacteria, viruses, fungi and protists

Antigen — a unique protein on the surface of a cell that allows the immune system to identify it as self or non-self

Antibody — a protein produced by lymphocytes that is complementary to a specific antigen and destroys or neutralises the pathogen

Phagocytosis — the process by which white blood cells engulf and digest pathogens

Lymphocyte — a type of white blood cell that produces antibodies specific to particular antigens

Antitoxin — a protein produced by lymphocytes that neutralises toxins released by pathogens

Vaccination — the introduction of small quantities of dead or inactive forms of a pathogen to stimulate immunity

Herd immunity — when a large proportion of a population is vaccinated, reducing the spread of disease and protecting unvaccinated individuals

Core concepts

Non-specific defence systems

The body has several defence mechanisms that work against all pathogens, not just specific ones. These are the first line of defence and work continuously to prevent infection.

The skin acts as a physical barrier preventing pathogen entry. The outer layer consists of dead cells that are difficult for pathogens to penetrate. If the skin is cut or damaged, blood clots form to seal wounds and prevent pathogen entry.

The nose produces mucus that traps particles containing pathogens. Tiny hairs (cilia) also prevent particles from entering the respiratory system.

The trachea and bronchi are lined with ciliated epithelial cells. These cells have hair-like projections (cilia) that waft mucus containing trapped pathogens upwards towards the throat where it is swallowed. The mucus is produced by goblet cells.

The stomach produces hydrochloric acid which maintains a pH of approximately 2. This acidic environment kills most pathogens that enter via the mouth in food, drink or mucus from the respiratory system.

These defence systems are described as non-specific because they do not distinguish between different types of pathogens — they work against all microorganisms that attempt to enter the body.

The immune system response

When pathogens enter the body and cause infection, the immune system responds. This involves white blood cells, which are produced in bone marrow and travel in the blood.

White blood cells detect pathogens by identifying their antigens as 'non-self' — different from the body's own cells.

There are two main types of white blood cell response you need to know:

Phagocytes carry out phagocytosis:

• They detect chemicals released by pathogens
• They move towards the pathogen
• They engulf (surround) the pathogen
• They release digestive enzymes to break down the pathogen
• The pathogen is completely destroyed

This is a non-specific response as phagocytes attack any pathogen.

Lymphocytes produce antibodies in a specific immune response:

• They detect antigens on the surface of pathogens
• They produce specific antibodies complementary to these antigens
• The antibodies bind to the antigens
• This causes pathogens to clump together
• Phagocytes then destroy the clumped pathogens more easily

Each lymphocyte produces antibodies specific to one type of antigen. The antibody shape is complementary to the antigen, like a lock and key.

Lymphocytes also produce antitoxins that bind to and neutralise toxins (poisons) produced by bacteria. This prevents the toxins from damaging body cells.

Memory cells and immunity

After an infection, some lymphocytes remain in the blood as memory lymphocytes (memory cells). These cells 'remember' the specific antigen.

If the same pathogen enters the body again:

• Memory cells recognise the antigen immediately
• They rapidly produce large quantities of the correct antibodies
• The pathogen is destroyed before symptoms develop
• The person is immune to that disease

The secondary immune response is much faster and produces more antibodies than the primary response. This is why you normally only get diseases like chickenpox once.

The graph of antibody concentration over time shows:

• Primary response: slow increase, lower peak, pathogens may cause symptoms
• Secondary response: rapid increase, higher peak, pathogens destroyed before symptoms

This natural immunity can last for years or even a lifetime, depending on the disease.

Vaccination

Vaccination protects against serious diseases by exposing the immune system to safe forms of pathogens, triggering immunity without causing illness.

How vaccination works:

• Dead or inactive forms of a pathogen are injected into the body
• These contain antigens but cannot cause disease
• Lymphocytes detect the antigens as foreign
• Antibodies specific to these antigens are produced
• Memory cells are created and remain in the blood
• If the live pathogen enters the body later, memory cells trigger rapid antibody production
• The person is immune without having suffered the disease

Examples of diseases prevented by vaccination:

• Measles, mumps and rubella (MMR vaccine)
• Polio
• Tetanus
• Tuberculosis (TB)
• Hepatitis B
• HPV (human papillomavirus)
• Influenza (flu vaccine, updated annually)

In the UK, children receive vaccinations according to the NHS childhood vaccination schedule, starting from 8 weeks old. The COVID-19 pandemic demonstrated how vaccination programmes can be rapidly developed and deployed globally.

Benefits and risks of vaccination

Benefits:

• Prevents individuals from developing serious diseases
• Reduces spread of communicable diseases in populations
• Achieves herd immunity when a high percentage are vaccinated
• Protects vulnerable people who cannot be vaccinated (babies, immunocompromised individuals)
• Has led to eradication of smallpox worldwide
• Reduces deaths and disability from infectious disease

Risks:

• Small risk of adverse reactions (typically mild, like soreness or fever)
• Very rare risk of allergic reactions
• Dead or inactive pathogens may not trigger as strong an immune response as live infection

The benefits vastly outweigh the risks for the overwhelming majority of people. Public health authorities continuously monitor vaccine safety.

Herd immunity

When a large proportion of a population (typically 90-95% depending on the disease) is vaccinated, the disease cannot spread easily. This protects:

• People who cannot be vaccinated (newborn babies, those with weakened immune systems)
• People for whom vaccines were ineffective
• The whole community by reducing disease transmission

In Caribbean countries and the UK, vaccination programmes aim to achieve herd immunity for serious diseases. If vaccination rates drop, outbreaks can occur — as seen with measles in some UK communities where vaccination rates fell.

Antibiotics and the treatment of disease

Antibiotics are medicines that kill bacterial pathogens inside the body without harming body cells. Examples include penicillin and amoxicillin.

Important points about antibiotics:

• They are effective against bacteria only
• They do NOT work against viruses (including colds, flu, COVID-19)
• Different antibiotics kill different types of bacteria
• They work by damaging bacterial cell walls or interfering with bacterial cell processes

Antibiotic resistance is a growing problem:

• Random mutations in bacterial DNA can lead to antibiotic resistance
• When antibiotics are used, resistant bacteria survive and reproduce
• Non-resistant bacteria are killed
• Resistant strains spread (e.g., MRSA — methicillin-resistant Staphylococcus aureus)
• These infections are difficult or impossible to treat

Reducing antibiotic resistance:

• Only use antibiotics when necessary
• Always complete the full course of antibiotics
• Doctors should not prescribe antibiotics for viral infections
• Development of new antibiotics (though this is slow and expensive)
• Improved hygiene in hospitals to prevent spread

You do NOT need to memorise specific antibiotic names beyond understanding examples like penicillin.

Worked examples

Example 1: Explaining phagocytosis (4 marks)

Question: Describe how phagocytes destroy pathogens that enter the body.

Mark scheme answer:

1. The phagocyte detects chemicals produced by pathogens (1 mark)
2. The phagocyte moves towards and engulfs the pathogen (1 mark)
3. The pathogen is taken inside the phagocyte in a vacuole (1 mark)
4. Enzymes are released that digest/break down the pathogen (1 mark)

Examiner note: Use correct terminology like "engulf" rather than "eat". Each stage of the process earns a mark, so structure your answer chronologically.

Example 2: Vaccination and immunity (6 marks)

Question: Explain how vaccination prevents a person from becoming ill if they are infected with a pathogen.

Mark scheme answer:

1. Vaccination introduces dead or inactive pathogens into the body (1 mark)
2. These pathogens carry antigens (1 mark)
3. Lymphocytes recognise these antigens as foreign (1 mark)
4. Lymphocytes produce antibodies specific to these antigens (1 mark)
5. Memory lymphocytes remain in the blood (1 mark)
6. If the live pathogen enters later, memory cells rapidly produce antibodies / mount a secondary response, so the pathogen is destroyed before symptoms develop (1 mark)

Examiner note: This is a 6-mark question requiring detailed explanation. Include the role of memory cells and explain why the person doesn't become ill.

Example 3: Antibiotic resistance (4 marks)

Question: Explain how antibiotic resistance develops in bacteria.

Mark scheme answer:

1. Random mutations occur in bacterial DNA (1 mark)
2. Some mutations make bacteria resistant to antibiotics (1 mark)
3. When antibiotics are used, non-resistant bacteria are killed (1 mark)
4. Resistant bacteria survive, reproduce and pass on the resistance gene / allele (1 mark)

Examiner note: The question asks you to "explain" so you must give reasons and use causal language like "so" or "therefore". Mention that mutation is random.

Common mistakes and how to avoid them

• Confusing antibodies and antibiotics. Antibodies are proteins made by lymphocytes; antibiotics are medicines. Antibodies work against specific antigens; antibiotics kill bacteria but not viruses.

• Saying vaccines contain "weakened" pathogens. Use the term "dead or inactive" as specified in the AQA specification. Some vaccines use weakened forms, but this level of detail isn't required at GCSE.

• Forgetting that antibiotics don't work on viruses. This is a very common exam question. Viruses reproduce inside body cells, so antibiotics cannot reach them without damaging our own cells.

• Not explaining why memory cells prevent disease. Don't just say "memory cells remember the pathogen." Explain that they rapidly produce antibodies in large quantities, destroying the pathogen before symptoms develop.

• Mixing up specific and non-specific responses. Phagocytosis is non-specific (works on all pathogens). Lymphocyte antibody production is specific (works on one type of antigen only).

• Incomplete explanations of antibiotic resistance. Always mention mutation is random, that resistant bacteria survive when antibiotics are used, and that they reproduce to pass on resistance.

Exam technique for "Human defence systems against disease"

• Command word awareness: "Describe" requires you to state features or processes. "Explain" requires reasons using linking words like "because", "so", "therefore" or "this causes". For example, "explain how" vaccination works needs causal links, not just a list.

• Mark allocation guides detail: A 1-mark question needs one clear point. A 6-mark question needs 6 distinct points with detailed explanation. Use the marks available to judge how much detail to include.

• Use correct scientific terminology: Write "engulf" not "eat", "lymphocyte" not "white blood cell" when being specific, "complementary" not "matching" for antigen-antibody binding. Precise language earns marks.

• Extended response questions: Structure longer answers logically (chronological order for processes). Include key terms like antigen, antibody, lymphocyte, memory cells. Link sentences to show understanding of cause and effect.

Quick revision summary

The body has non-specific defences including skin, stomach acid, and phagocytosis. The specific immune response involves lymphocytes producing antibodies complementary to antigens on pathogens. Memory cells provide immunity by rapidly producing antibodies upon reinfection. Vaccination creates immunity without disease by using dead or inactive pathogens. Antibiotics kill bacteria but not viruses, and overuse causes antibiotic resistance through natural selection of resistant strains.